Previous investigations in this laboratory have shown that, under certain circumstances, antigen structure may critically determine the way in which an immune complex is handled in an animal. It is known that the presence of certain antibodies in patients with systemic lupus or related diseases define clinical categories of patients. With the purification of the antigen against which those antibodies are directed, it has become possible to ask if the structure of the antigen determines the behavior of the complexes. It is the purpose of these studies to explore the metabolism of immune complexes made from the anti-Sm antibodies, which can react with either of two distinct nuclear particles (U1 or U2-6), and to compare the metabolism of complexes of U1 particles with either anti-Sm antibodies or anti-RNP antibodies. So far, small quantities of U2-6 have been radiolabeled and been traced after injection into mice with or without auto-antibodies, either exogenously added or endogenously produced (MRL/1 mice).